The present invention relates to a sheet of lead frames for use in the assembly of semiconductor packages. In particular, the present invention relates to a sheet of lead frames that has integrally formed spacing members for spacing an underside of the sheet from a planar support such as a heating block.
Typical semiconductor packages are formed with a one or more semiconductor dice mounted to a lead frame. The lead frame includes an arrangement of external connection pads (lead fingers) and one or more flags. Semiconductor dice are attached to respective ones of the flags and electrical connections of each dice are electrically coupled, typically by bonded wires, to the external connection pads. The lead frame is normally one of many lead frames that are integrally formed in a conductive sheet by punching or etching. After each semiconductor package is separately encapsulated with an encapsulation material, each of the lead frames is separated (singulated) from all other lead frames in the conductive sheet by a cutting or punching process.
When considering semiconductor packages such as Power Quad Flat No Leads (PQFN) packages and Power Dual Flat No Leads (PDFN) packages, the semiconductor dice have a metallic base and the dice are attached to their respective flags by a solder. The solder is pasted onto the flags and then the semiconductor dice are placed on the solder. The conductive sheet of lead frames is then placed onto a heating block in an oven. The oven heats the solder, causing it to melt, which bonds the semiconductor dice to their respective flags. Unfortunately, when the solder is heated in the oven, some solder may be splashed onto the heating block, which results in solder globules landing on the heating block. These solder globules may attach to undersides of subsequent conductive sheets that are placed on the heating block thereby causing potential shorts, package encapsulating defects and uneven connection pad mounting surfaces. Thus, it would be advantageous to have a way to prevent such shorts, defects and uneven mounting surfaces.